1. Technical Field
This invention relates generally to electronic imaging devices, and in particular, to measuring dark signals in electronic imaging devices.
2. Related Art
Conventional solid-state image sensor, such as a CMOS image sensor, typically has an array of pixels arranged in an active area. Each pixel in the active area has a radiation detector, such as a photodiode, to sense radiation intensity. During an integration period the radiation intensity is sampled and a charge is read out of the radiation detector as an electrical signal by an associated image processor. CMOS image sensors typically have a dark signal level or voltage offset that occurs primarily from low levels of junction leakage. The exact extent of junction leakage can vary with slight changes in manufacturing conditions causing differences in the expected output signal under conditions of zero illumination (darkness). Changes in operating condition such as integration time or temperature may also cause the value of the dark signal level to vary in the CMOS image sensor.
Knowledge of the dark signal level, sometimes referred to as “black reference level”, is useful in the reconstruction of images captured by the CMOS image sensors. Conventional image sensors often measure current leakage or the dark signal level during an integration period in order to provide a black reference level. Selected pixels in the solid-state image sensor are covered with an opaque material during fabrication to prevent radiation (i.e. light) from directly striking the photo detector. The application of a simple light shield approach during fabrication suffices to establish a dark reference level during an integration period under some conditions.
A problem exists with utilizing the simple light shield approach when a bright source of radiation, such as a light bulb, sun or long wavelength radiation, such as red light having a wavelength of 600 to 680 nanometers, illuminates the edge of the active area in the solid-state imager sensor. Radiation from bright sources near the edge of the active area is able to penetrate laterally and relatively deep into the semiconductor substrate of the solid-state image sensor creating charge carriers (i.e. electron-hole pairs). While the electron holes may diffuse to a substrate isolation terminal (i.e. ground), the minority carriers (electrons) often diffuse to neighboring covered or otherwise shielded pixels. The diffusion of the minority carriers results in an increase in the measured dark reference level. This increase in dark reference level (also referred to as cross-talk) results in a false or inaccurate black reference level, which in turn, adversely affects the detected image quality.
A conventional approach to reducing the cross-talk problem in a solid-state image sensor requires dummy pixels between active area pixels and shielded pixels. The dummy pixels act to isolate the shielded pixels that measure the dark reference level from the diffusion of the minority carriers. A single column of dummy or isolation pixels, however, is generally insufficient to prevent cross-talk from occurring. Multiple columns and rows of isolation pixels are needed to correct for gaps or insufficient depth of the photo-detector in each isolation pixel and may require a relatively significant amount of area in the solid-state imager resulting in larger die sizes. Furthermore, the type and amount of light shield material utilized to cover the dark signal detectors (i.e. colored photoresist material) also increase the cost and complexity of fabrication. Thus, there is a need in the art for measurement of the dark signal reference without significantly increasing the cost, die size and complexity of a solid-state image sensor.